In the cultivation water of culturing farms, cultivating farms and aquariums, ammonia contained in excrements from fishes/shellfishes, residues of feeds, or the like are decomposed in water by microorganisms, with the result that ammonia is left in the water, and the ammonia is accumulated at the lapse of time. Since ammonia is high in biological toxicity and harmful to fishes/shellfishes, as countermeasures against this problem, it has been known that, for example, ammonia is removed by using a removing method by biodegradation utilizing microorganisms. In the aquaculture method for removing ammonia by using such a biological treatment, however, problems are raised in that a large area is required as a treatment tank, or large power is required for an aeration pump to cause high costs, or the ammonia decomposition treating performance becomes unstable due to influences of temperatures and amounts of loads.
For these reasons, in recent years, a decomposing treatment has been proposed in which ammonia is removed by using a physical treatment in which ozone or electrolyzed hypochloric acid is used. In this decomposing treatment, since oxidizing substances harmful to fishes/shellfishes, referred to as oxidants, are easily accumulated as the treatment proceeds, various methods for removing ammonia, while suppressing the generation of oxidants, or eliminating oxidants, have been disclosed.
As the removing method of ammonia by using the physical treatment of this type, for example, Patent Document 1 has disclosed an ammonia decomposing method by using ozone. This Patent Document has described a structure in which an oxidation-reduction potential measuring electrode for measuring an oxidation-reduction potential of seawater that has been subjected to an ozone treatment is installed near an outlet of an ozone contact tower so that the ozone generation amount is controlled so as to set its oxidation-reduction potential within a range of 450 to 650 mV (see Paragraph [0005]). This ammonia decomposing method is designed so as to decompose ammonia by utilizing the fact that as the decomposition of ammonia proceeds by ozone injection and at the time when the ammonia becomes substantially zero, oxidants are generated together with an abrupt increase of the oxidation-reduction potential.
On the other hand, Patent Document 2 has disclosed a fishes/shellfishes rearing water purifying device in which by utilizing hypochloric acid generated in cultivation water by the use of electrolysis, ammonia is decomposed. In this Patent Document 2, it is disclosed that by detecting a hypochloric acid concentration during the rearing process, a controlling process is carried out so as to reduce the residual chlorine concentration of cultivation water (see Paragraph [0009]). It has been known that the hypochloric acid to be utilized in this water purifying device is one kind of oxidants, which is a very toxic substance to fishes/shellfishes.
Patent Document 3 has disclosed an ammonia decomposing device in which electrolyzed hypochloric acid is used. This Patent Document 3 describes a process in which ammonia and hypochloric acid discharged by fishes/shellfishes are adsorbed onto the surface of an adsorbing material so as to remove ammonia (see Paragraph [0015]).